Martina Morris, a sociologist at the University of Washington, has shown that long-term concurrency is more of a public health danger than serial monogamy because it permits HIV and other sexually transmitted diseases to spread to others quickly, rather than confining them in a single relationship for months or years.

Lockheed's latest run-in with the Pentagon sees defense chiefs trying to tie additional funding for the next batch of F-35s to a formula involving a so-called concurrency clause, where the cost of additional work discovered during aircraft testing would fall to suppliers.

But he also announced delays in production to undo the costly refits required by so-called "concurrency" in the F-35 program - in other words, early copies of the plane are being produced by contractor Lockheed Martin Corp. even as the design in being worked out.

To be of any use, however, it must be possible to structure the code to expose and later exploit the concurrency and permit the sub problems to actually run concurrently; that is, the concurrency must be exploitable.